Generally, a hydraulic excavator as a typical example of construction machines is largely constituted by an automotive lower traveling structure, an upper revolving structure which is swingably mounted on the lower traveling structure by means of a revolving ring, and a working mechanism provided on the front side of the upper revolving structure. A swing device is provided between the lower traveling structure and the upper revolving structure, and as this swing device is operated, the upper revolving structure is adapted to revolve on the lower traveling structure.
In this instance, the swing device is generally comprised of a reduction gear which is mounted on the upper revolving structure to decelerate an input rotation and output a decelerated rotation, a revolving motor which is provided on an upper side of the reduction gear to input the rotation of its motor shaft to the reduction gear, and an output shaft for outputting to the revolving ring the rotation of the motor shaft with its speed reduced by the reduction gear.
Here, a so-called hybrid type swing device has been proposed in which a hydraulic motor and an electric motor are arranged to be used in common as the revolving motor. According to this hybrid type swing device, as the output shaft is rotatively driven by both the hydraulic motor and the electric motor, it is possible to reduce exhaust gas from the engine serving as a driving source of the hydraulic motor (Patent Document 1).